This invention relates to semiconductor processing and, more particularly, to a process for patterning a metallization layer using a deep UV photo-resist.
In the fabrication of integrated circuit devices, one or more metallic layers, typically aluminum or titanium are deposited and subsequently patterned to provide contacts and/or electrical connections between various circuit elements. Conventionally, a photoresist is deposited over the metallic layer and then exposed to a light pattern and developed. If the metallic layer is aluminum, the metal of choice, then it is selectively plasma etched with a chlorine-containing gas through openings in the resist layer. The remaining photoresist is then removed leaving the desired metallic pattern.
Various coatings have been used over the reflective aluminum or titanium surfaces to improve resist patterning control. These coatings which are antireflective, reduce interference effects and diffuse scattering. They are particularly effective if the illumination is monochromatic. However, the effects have become less tolerable as line width and pitch have reduced with newer generations of integrated circuit configurations having greater density.
In the past, silicon has been the material of choice for use as an antireflective cap. However, silicon has a potential to diffuse into the aluminum metallization at temperatures in the order of 200.degree. C. That temperature range is typically achieved in dry plasma photo rework processes, post-etch resist stripping, and metal annealing. Silicon diffusion results in loss of Al/Cu integrity. It thus effectively eliminates the possibility of photo rework due to the loss of the antireflective coating. Given these difficulties with the use of silicon as a cap material, titanium nitride has been employed as a substitute. It is readily sputtered in situ following the sputtering of the Al/Cu/Si layer.
Reference is made to U.S. Pat. No. 4,820,611 which describes the use of titanium nitride as an antireflective coating in a photolithographic process. In the '611 patent, the process comprises the steps of interposing a layer of TiN between a metal layer, typically aluminum, and a resist layer. This is done to reduce the amount of light reflected back from the metal surface into the resist during exposure.
Other techniques have been described in the literature to improve patterning by the use of antireflective layers. U.S. Pat. No. 3,884,698 employs an antireflective layer that it is interposed between an insulating layer, typically aluminum oxide, and a resist to promote uniform exposure of the resist. In this system uniform exposure of the resist occurs by eliminating reflected light from the interface between the insulating layer and the substrate. The antireflective layer may be a metal such as molybdenum or tantalum nitride.
U.S. Pat. No. 4,587,138 relates to a technique for forming a patterned conductive layer by employing a low temperature spin on glass containing a dye which is used as an antireflective coating between a resist and a conductive layer such as aluminum or aluminum/silicon.
In these prior art techniques, the antireflective layer is employed in contact with the resist. This in turn creates problems heretofore unsolved in the art particularly as device density increases into the submicron regime. At those density levels, it is necessary to employ deep-UV photolithographic imaging employing wavelengths in the order of 250 nm to achieve the required resolution. There are few resist systems which exist that are capable of printing reliable images of less than 0.7 micron. Such resist systems while working acceptably over oxide and silicon surfaces do not perform satisfactorily when placed in direct contact with a nitride layer. Thus, contemporary resist systems can be used in combination with nitride layers for only selective processing.
For example, many levels are compatible with such contemporary resist systems, but other levels are not. The preferred resist system is an acid-catalyzed system, that is, one where acid groups are generated during exposure of the resist to radiation of a wavelength sufficient to initiate the photochemical reaction. An example of an acid-catalyst system is found in U.S. Pat. No. 4,491,628. When using such an acid-catalyzed resist system, photolithographic patterning utilizing a TiN cap is unsatisfactory. Specifically, at those levels "webbing" or joining at the base of the resist structures is observed. Such a defect does not dissolve with satisfactory structure definition even when excess developing is employed. It is believed that this defect is a consequence of chemical interaction between the TiN surface and the base of the photoresist layer. The result of this interaction renders the bottom portion of the resist immune to either deep UV photoimaging or image developing operations. Thus, this failure to develop the resist causes defects, such as "webbing" between the photoresist features.